Changeling

The boy had wandered into the woods. He was lost, probably on purpose, by his parents, and had followed the inaudible music of the faêrie, and had found passage into their realm. It had caused a bit of a stir in the court, but Titania had found the boy amusing, charming even.

— Titania and the Changeling - Maxime Teppe

Changelings is a term that refers to mortals that have spent time with the fey and have become somehow affected by this experience. Changelings can behave in strange and eerie ways, they often possess sorcerous gifts and they can even have an otherworldly appearance, e.g. eyes glowing with magic. Most changelings are taken by the fey as children -- by abduction, for example, or as bargain -- and raised by the fey to adulthood, but there are also tales of knightly champions and great adventures being lost in Tír-na-nÓg only to return many years later, bewitched and transformed.
Changelings rarely, if ever, return to their past lives, and instead function as servants to their fey masters. And even if they were to be reunited with their loved ones, folk tales state that a changeling would not even recognize his family, instead treating them with complete disinterest and usually responding to any interactions with silence.
Changelings behave most often in a very stoic and patient manner. They speak only little, betraying the motives of their masters as little as possible. Peasants superstitions abound of changelings sharing permanent telepathic bonds with the fey, functioning as spies and informants that can be stopped only with death or banishment.

The Princess and the Trolls by John Bauer